The present invention relates to RF broadcasting and specifically to a digital television transmitting system having a circuit for recovering data pulses and transport clock pulses from a serial data pulse stream.
In the United States, the Federal Communications Commission (FCC) has established guidelines for broadcasting television signals. Digital television signals (DTV) as well as analog NTSC signals will be simultaneously broadcast for a period until approximately the year 2006, after which only DTV signals will be broadcast.
In a proposed DTV system, and as shown in FIG. 1, a signal bearing video and audio data may be sent from a signal source, such as a television studio to a RF/Transmission site which may not be co-located therewith. In such a system, the video and audio data signal may be transmitted (or transported) from the studio to the RF/Transmission site using conventional communications techniques, such as microwave links. This signal, often referred to as the Transport Signal, will contain both the data and the clock for the data. The frequency of the transport clock has been established as being 19.39 MHz for the DTV known as the 8 VSB format. The data to be transported at this frequency is a pulse stream of non-return to zero (NRZ) data. It is proposed that the NRZ data and the transport clock be combined to form a single serial pulse stream employing biphase-mark coding as will be discussed hereinafter. This entails providing an interface clock double that of the transport clock to provide the biphase-mark serial data pulse stream.
The present invention is directed toward a recovery circuit for recovering the NRZ data pulses and the transport clock pulses from the biphase-mark serial data pulse stream.
In accordance with the present invention a digital television transmitting system is provided having a recovery circuit for recovering non-return to zero (NRZ) data pulses and transport clock pulses from a biphase-mark serial data pulse stream. The recovered NRZ data pulses and transport clock pulses were previously combined into the biphase-mark serial data pulse stream. The recovered NRZ data pulses and the transport clock pulses are intended to be applied to modulation and amplifying circuitry for broadcasting by an antenna.
The recovery circuit includes a first circuit that receives the biphase-mark serial data pulse stream and provides therefrom a train of first clock pulses such that the rising edge of each biphase-mark serial data pulse corresponds with the rising edge of one of the first clock pulses. A second circuit receives the biphase-mark serial data pulses and the first clock pulses and provides therefrom a train of de-serialized data pulses each having a rising edge corresponding with a rising edge of one of the first clock pulses. A third circuit receives the de-serialized data pulses and the first clock pulses and converts the de-serialized data pulses into recovered NRZ data pulses.
In accordance with a more limited aspect of the present invention an inverter receives the first clock pulses and provides therefrom a train of second clock pulses of the same frequency and which are inverted from that of the first clock pulses.
In accordance with a still further aspect of the present invention a clock divider receives the first clock pulses and provides therefrom a train of third clock pulses and which third clock pulses are at a frequency one-half that of the first clock pulses.
In accordance with a still further aspect of the present invention a second inverter receives the third clock pulses and provides therefrom a train of fourth clock pulses of the same frequency as the third clock pulses but inverted therefrom and with the fourth clock pulses serving as the recovered transport clock pulses.
In accordance with a still further aspect of the present invention a data retimer receives the recovered NRZ data pulses and shifts the recovered NRZ data pulses so that the rising edge of each recovered transport clock pulse is located between the rising edge and falling edge of each the NRZ data pulse.